Method of preparing a flexographic printing master

ABSTRACT

A method of preparing a flexographic printing master including an optional elastomeric floor, an optional mesa relief, and an image relief are applied in this order on a flexographic printing support includes applying and curing fluid droplets thereby building up a plurality of layers of fluid on top of each other, wherein each fluid droplet applied is at least partially cured before an adjacent fluid droplet is subsequently applied, with the exception that a fluid droplet applied during building up at least one layer of fluid is not cured before an adjacent fluid droplet of the same layer is subsequently applied.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 National Stage Application ofPCT/EP2011/057946, filed May 17, 2011. This application claims thebenefit of U.S. Provisional Application No. 61/346,475, filed May 20,2010, which is incorporated by reference herein in its entirety. Inaddition, this application claims the benefit of European ApplicationNo. 10163064.8, filed May 18, 2010, which is also incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of making a flexographicprinting master by inkjet.

2. Description of the Related Art

Flexography is today one of the most important printing techniques andis commonly used for high-volume runs. Flexography is used for printingon a variety of substrates such as paper, paperboard stock, corrugatedboard, films, foils and laminates. Coarse surfaces and stretch films canonly be economically printed with flexography, making it indeed veryappropriate for packaging material printing.

Today flexographic printing masters are prepared by both analogue anddigital imaging techniques. Analogue imaging typically uses a film maskthrough which a flexographic printing precursor is exposed. Digitalimaging techniques include:

-   -   Direct laser engraving as disclosed in e.g. EP-As 1710093 and        1936438;    -   UV exposure through a LAMS mask wherein LAMS stands for Laser        Ablative Mask System as disclosed in e.g. EP-A 1170121;    -   Direct UV or violet exposure by laser or LED as disclosed in        e.g. U.S. Pat. No. 6,806,018; and    -   Inkjet printing as disclosed in e.g. EP-As 1428666 and 1637322.

EP-A 1428666 discloses a method of making a flexographic printing masterby means of jetting subsequent layers of a curable fluid on aflexographic support. Before jetting the following layer, the previouslayer is immobilized by a curing step.

U.S. Pat. No. 6,520,084 also discloses a method of preparingflexographic printing masters using inkjet. In this method, a removablefiller material is used to support the relief image being printed andthe relief image is grown in inverted orientation on a substrate.Disadvantages of this method are the removal of the filler material andthe release of the relief image from the substrate. In U.S. Pat. No.7,036,430 a flexographic printing master is prepared by inkjet whereineach layer of ink is first jetted and partially cured on a blanketwhereupon each such layer is then transferred to a substrate having anelastomeric floor, thereby building up the relief image layer by layer.US20080053326 discloses a method of making a flexographic printingmaster by inkjet wherein successive layers of a polymer are applied to aspecific optimized substrate. In US20090197013, also disclosing aninkjet method of making a flexographic printing master, curing means areprovided to additionally cure, for example, the side surfaces of theimage relief being formed.

The major advantage of an inkjet method for preparing a flexographicprinting master is an improved sustainability due to the absence of anyprocessing steps and the consumption of no more material as necessary toform a suitable relief image (i.e. removal of material in the nonprinting areas is no longer required).

A problem however that may occur with these inkjet methods is the lackof smoothness of the at least partially cured layers of fluid. Such lackof smoothness may be passed on from layer to layer forming the reliefimage or may even be reinforced as more layers are jetted on top eachother and may result in an unsmooth printing surface of the relief,which can give rise to several printing artifacts such as a deficientreproduction of highlight dots or a deficient rendering of solids. Foroptimal printing performance, it is required that flexographic printingmasters have a relief image with a printing surface that is sufficientlyflat or even.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide an inkjet method of preparing aflexographic printing master wherein the obtained flexographic printingmaster is characterized by a sufficiently flat or even printing surfaceso as to obtain optimal printing properties.

A preferred embodiment of the present invention is achieved by a methodof preparing a flexographic printing master as defined below. Furtherpreferred embodiments of that method are also described below.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a preferred embodiment of an apparatus for printing aflexographic printing master on a cylindrical sleeve.

FIG. 2 shows a different view of a preferred embodiment of an apparatusfor printing a flexographic printing master on a cylindrical sleeveshowing the simultaneously printing of several fluid layers.

FIG. 3 shows a cross section of a preferred embodiment of theflexographic printing master wherein the relief image comprises a “tophat” profile.

FIG. 4 shows a cross section of another preferred embodiment of theflexographic printing master wherein the relief image comprises a“regular” profile.

FIG. 5 shows how multiple layers are printed on a rotating sleeve duringa single pass.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a method of preparing a flexographic printing master according to apreferred embodiment of the present invention an optional elastomericfloor (500), an optional mesa relief (600) and an image relief (700) areapplied in this order on a flexographic printing support (1) by applyingand curing fluid droplets thereby building up a plurality of layers offluid on top of each other characterized in that each fluid dropletapplied is at least partially cured before an adjacent fluid droplet issubsequently applied, with the exception that a fluid droplet appliedduring building up at least one layer of fluid is not cured before anadjacent fluid droplet of the same layer is subsequently applied.

From 2D image to 3D relief

The image to be printed can be any digital image represented as a rasterbitmap. A typical image comprises multiple objects such as photographs,graphic objects and text objects. These objects are usually representedusing a page description language and are rendered into a digital imageby a raster image processor (RIP) such as made available by the companyAdobe Systems Incorporated. The image can be monochrome or coloured. Inthe latter case the colour image is first separated into a set of inkseparations that correspond with a set of corresponding printing inks.

Halftoning refers to an image processing technique which enables imageshaving multiple densities to be rendered with a system having restricteddensity resolution. For example, a digital image that has pixels with adensity resolution of 8 bits (256 shades) has to be rendered on a binaryprinting system having only two shades of density corresponding with inkor no ink. Halftoning can be AM (amplitude modulation), FM (frequencymodulation) or XM (hybrid halftoning).

The two dimensional (2D) image to be printed with the flexographicprinting master has to be converted into a three dimensional (3D) reliefimage that in a preferred embodiment of the present invention has to beprinted on a flexographic support by inkjet. The 2D image to be printedcorresponds in fact with the top layer or printing surface of the reliefimage. This top layer however has to be supported by the other layersforming the relief.

EP-A 1437882 teaches an image processing method for creating such a 3Drelief image starting from the 2D image to be printed. A binaryhalftoned digital image represents the printing surface or top layer ofthe relief image. A topographic operator, such as a circular symmetricsmoothing filter, is then applied on this binary halftoned imageresulting in a contone image of which the densities represent theheights of a relief print master. The contone image is then conceptuallysliced to obtain intermediate binary layers which, when printed on topof each other, form a 3D printing master. The effect of the smoothingfilter is that around each pixel in an upper intermediate layer a circleof identical pixels is replicated in a lower intermediate layer. As aresult, every lower intermediate layer always entirely supports anyupper intermediate layer.

A possible disadvantage with the image processing technique disclosed inEP-A 1437882 is that it requires many computations. In EP-A 2199065 animage processing technique, also for creating a 3D printing masterstarting from a binary halftoned digital image, is disclosed whichrequires less computations. The method takes advantage of theobservation that the exact shape of the intermediate layers for creatinga 3D print master is not very important as long as the condition isfulfilled that every lower intermediate layer supports the higherintermediate layers.

Printing the 3D Relief on a Support

Once the 3D relief has been calculated, it has to be physicallyreconstructed by a 3D printing apparatus on a flexographic support. Forexample, the inkjet printing method as disclosed in EP-A 1428666 can beused to print the 3D relief. In this method a flexographic printingmaster is formed by applying subsequently on a flexographic support atleast two layers of polymerisable fluid with an inkjet printer. In thisinkjet method, a previously applied layer is at least partially curedbefore applying a subsequent layer.

FIG. 1 shows a preferred embodiment of an apparatus 100 for printing aflexographic printing master on a cylindrical sleeve 130. 140 is arotating drum that is driven by a motor 110. A printhead 160 moves in aslow scan direction Y parallel with the axis of the drum at a linearvelocity that is coupled to the rotational speed X of the drum. Theprinthead jets droplets of a polymerisable fluid onto a removable sleeve130 that is mounted on the drum 140. These droplets are gradually curedby a curing source 150 that moves along with the printhead and provideslocal curing. When the flexographic printing master has been printed,the curing source 170 provides an optional and final curing step thatdetermines the final physical characteristics of the flexographicprinting master. It is however also possible to use only one curingsource, for example the curing source 170. The location of this curingsource 170 with respect to the printhead 160 and the rotational speed ofthe rotating drum 140 then determine the time between applying andcuring the fluid droplets.

The 3D image representing the relief to be printed can thus berepresented in X, Y and Z dimensions, whereby the X dimensioncorresponds with a fast scan orientation of a printing device, the Ydimension with a slow scan orientation and the Z dimension with theorientation of the relief features of the print master. The 3D image canbe subdivided into a top layer, which corresponds with the image to beprinted with the flexographic printing master, and supportingintermediate layers parallel to the X and Y dimensions.

The volume, speed and direction of fluid droplets ejected by the nozzlesof an inkjet printhead may slightly vary between individual nozzles. Itis well known in 2D inkjet printing that in absence of any compensatingmeasures, such as shingling and interlacing techniques, this may lead toimage quality artifacts such as banding and streaking which arecorrelated with differences between individual nozzles.

Such image quality artifacts may also appear in 3D inkjet printing.

To minimize such quality artifacts, the flexographic 3D image ispreferably formed in accordance with the method disclosed in the EP-A2199066. With this method, the layers making up the relief image areprinted in such a way that at least two adjacent pixels in the Zdimension are printed with different nozzles. This achieves the effectthat image quality artifacts correlated to a specific nozzle arespatially diffused in the Z dimension. The image quality artifactsrelated to a specific nozzle are also decorrelated in the X and Ydimensions by avoiding that neighbouring pixels along the X and Ydimensions are printed by the same nozzle.

According to a preferred embodiment, multiple layers of the relief imageare simultaneously printed by different sets of nozzles of the sameprinthead. For example, fluid droplets of a lower intermediate layer areprinted by a first set of nozzles at a first location of the printingmaster and are at least partially cured. At the same time, fluiddroplets of an upper intermediate layer are printed on top of alreadyprinted and at least partially cured droplets of the lower intermediatelayer by a second set of nozzles of the same printhead at a secondlocation. FIG. 2 shows a different view of a preferred embodiment of anapparatus (200) for printing a flexographic printing master on acylindrical sleeve showing the simultaneously printing of several fluidlayers. FIG. 2 demonstrates that, as the printhead 210 moves from leftto right in the direction Y, droplets 250 are jetted onto the sleeve240, whereby the “leading” part 211 of the printhead 210 prints dropletsthat belong to a lower layer 220, whereas the “trailing” part 212 of theprinthead 210 prints droplets of an upper layer 230.

FIG. 5 is based on the upper part of FIG. 13J of EP-A 2199066. It showsa portion of the pixel positions on a drum upon which inkjet dropletsare jetted by a printhead 500 according to a preferred embodiment of theinvention that is disclosed in this document. The arrow 501 indicatesthe movement of the printhead in the fast scan direction relative to thedrum. The arrow 502 indicates the direction of the slow scan directionof the printhead relative to the drum. The distance 503 corresponds withthe slowScanPitch, i.e. the distance that the printhead travels in theslowscan direction during one revolution of the drum. The tickmarks 504correspond with positions in the fast scan direction at which theprinthead 500 can eject droplets within a specific single revolution.The numbers in FIG. 5 indicate the positions at which during a givenrevolution the printhead has deposited droplets. For example, the pixelsindicated with “1” correspond with the positions where a droplet wasejected during a first revolution, whereas the pixels indicated with “2”correspond with the positions where a droplet was ejected during asecond revolution. Because of the relationship between the firingfrequency, the rotational speed of the drum, the slowScanPitch and thenozzlepitch of the printhead 500, a pattern 506 of 3 by 3 pixels withina layer is filled up after exactly nine revolutions. During a tenthrevolution, a first pixel position of such a filled up pattern receivesa first droplet of a second layer. The study of FIG. 5 teaches thatdifferent nozzles of the same printhead are jetting droplets ondifferent layers. For example, on the right hand side the first dropletsare printed onto pixel positions of the lowest layer, whereas on theleft hand side, the first droplets are already jetted onto pixelpositions of the second layer. In the general case different nozzles ofthe same printhead may print simultaneously onto N different layers.

The droplets that are ejected during each revolution are partly cured bythe curing source 150 (FIG. 1). The effect of this is that according tothe preferred embodiment of the teachings of the application EP-A2199066, a droplet that is jetted onto the drum never touches a dropletthat has not received partial curing.

Elastomeric Floor

Before applying the optional mesa relief (600) and the image relief(700), the flexographic support is optionally provided with one or moreelastomeric layers, the latter making up the so-called elastomeric floor(500). To reduce the manufacturing time of a flexographic printingmaster and because resolution is not relevant while forming theelastomeric floor, the elastomeric floor, as well as the optional mesarelief, may be formed using fluid droplets having a drop volume which isat least 25% larger compared to the drop volume of the fluid dropletsused while printing the image relief on the elastomeric floor or on theoptional mesa relief. This may be achieved by using inkjet printerincluding a first and a second set of nozzles, wherein a nozzle diameterof a nozzle of the first set of nozzles is larger than a nozzle diameterof a nozzle of the second set nozzles. In making the printing relief onthe flexographic printing support, the first set of nozzles having alarger nozzle diameter is used for printing the elastomeric floor andthe optional mesa relief and the second set of nozzles is used forprinting the image relief.

According to one preferred embodiment of the present invention, theelastomeric floor is applied by the inkjet method defined below togetherwith an optional mesa relief and the image relief.

According to another preferred embodiment of the invention, theelastomeric floor may be applied by other coating techniques whereupon,after partially or fully curing the layers, the optional mesa relief andthe image relief are applied by inkjet. Such a method is disclosed inEP-A 2033778. WO2008/034810 and WO2010/003921 disclose a coating methodand coating device with which sleeves are provided with one or moreelastomeric layers. As the coating device has a limited floor space itis a preferred coating device to be used in combination with a preferredembodiment of the present invention.

The height of an elastomeric floor (500) applied on a flexographicsupport (1) is preferably between 0.3 mm and 2 mm.

Mesa Relief

A preferred method for forming a 3D relief of a flexographic printingmaster is disclosed in EP-A 2199082. In this method the relief includesa so-called “mesa relief” as shown by the flexographic printing masterin FIG. 3. The printing master in FIG. 3 comprises a support (1)whereupon an elastomeric floor (500) is applied. On the elastomericfloor (500), the mesa relief (600) and the image relief (700) areapplied. The mesa relief is only present in those parts of theflexographic printing master comprising image features such as text,graphics and halftone images. In extended areas where such imagefeatures are absent, there is no mesa relief. This makes it possible tominimize the amount of fluid necessary to form the flexographic relief.A mesa relief preferably has a height in a range from 50 μm to 1 mm, forexample 0.5 mm.

The mesa relief in different image areas of the flexographic printingmaster has preferably the same height. However, it is not necessary thatthe height of the mesa relief is identical over the whole flexographicprinting master.

The mesa relief is preferably applied on the support by the same inkjetapparatus that is used for applying the image relief. However, toincrease the manufacturing speed of the flexographic printing master andbecause resolution is not that relevant for the mesa relief, it ispreferred to use for printing the mesa relief fluid droplets having adrop volume which is at least 25% larger compared to the drop volume ofthe fluid droplets used for printing the image relief on the mesarelief. This may be achieved by using inkjet printer including a firstand a second set of nozzles wherein a nozzle diameter of a nozzle of thefirst set of nozzles is larger than a nozzle diameter of a nozzle of thesecond set nozzles.

Image Relief

The top layer (800) of the image relief corresponds with a halftonebitmap that defines the image to be printed by the printing master. Theuppermost layers of the image relief are preferably identical in shapeand size as the top layer, producing a vertical relief slope anddefining a “top hat segment” (FIG. 3, 750). Such a top hat may have aheight between 10 and 500 μm and preferably between 20 and 200 μm. Thevertical relief slope of a top hat segment has the advantage that theprinting surface remains constant during printing, even when pressurevariations occur between the printing master and the anilox roller orbetween the print master and the printable substrate, or when theprinting master wears off.

The intermediate layers, together forming a “sloped segment” (775), arepreferably printed with a slope having an angle α that is less than 90degrees. The angle can be between 25 and 75 degrees, preferably between40 and 60 degrees, for example 50 degrees. The angle α can be controlledby controlling the height of the individual layers, their number and thedifference in size between subsequent layers. Using a larger slope angleα (i.e. a steeper slope) has the advantage that small features on theprint master will suffer less from pressure variations during printing.

Alternatively however, it is also possible that the image relief has a“regular” profile, as shown in FIG. 4. Flexographic printing mastersmade by an analogue imaging technique such as a UV exposure through amask have a relief having such a “regular” profile. A relief having a“top hat” segment can only be made by laser engraving or by inkjetprinting. The relief having a “regular” profile as shown in FIG. 4comprises an image relief (700) on an optional mesa relief (600)previously printed on an elastomeric floor (500) provided on aflexographic support (1). The shoulder (850) of the image relief (700)has a slope with a slope angle α. This slope angle α can be optimized asdescribed above. A disadvantage of such a flexographic printing masterlies in the fact that when the upper layers of the image relief are worndown, dot gain occurs due to the sloped image shoulder.

When building up the 3D relief image using the printing method disclosedin EP-A 2199066, each fluid droplet applied for forming the 3D image isat least partially cured before an adjacent droplet of fluid of the samelayer is subsequently applied. This prevents the droplets to spread orto coalesce. However, when using such a method, a problem may be a lackof smoothness of the applied layers of fluid. This problem may becomemore pronounced when larger drops of fluid are used, for example toprepare the elastomeric floor and/or the optional mesa relief to reducethe manufacturing time of the flexographic printing master.

Such lack of surface smoothness may be passed on from layer to layerforming the relief or may even be reinforced as more layers are jettedon top each other and may therefore result in an unsmooth surface of therelief, which can give rise to several printing artifacts such as adeficient reproduction of highlight dots or a deficient rendering ofsolids.

A possible solution to this problem has been suggested in EP-A 2199081wherein after applying a relief image by inkjet, a grinding step isforeseen to ensure a flat surface. However, such an additional grindingstep prolongs the time necessary to prepare the master and results inadditional waste, thereby diminishing the overall sustainability of themethod. Another solution was proposed in U.S. Pat. No. 6,520,084 whereinthe relief image is grown in inverted orientation on a substrate toensure a smooth surface of the relief image. However, the separation ofthe relief image from the substrate is an additional step and removablefiller material is needed to support the inverted image relief.

It has now been found that the smoothness of the top surface, i.e. theprint surface, can be improved when a fluid droplet applied duringbuilding up at least one layer of fluid is not cured before an adjacentfluid droplet of the same layer is subsequently applied. The bestresults are obtained when all fluid droplets forming the layer are notcured before an adjacent fluid droplet of the same layer is subsequentlyapplied. However, satisfactory results are also obtained when at least75%, more preferably at least 90%, most preferably at least 95% of thetotal amount of fluid droplets forming the layer are not cured before anadjacent fluid droplet of the same layer is subsequently applied.Preferably, the fluid droplets that are not cured are homogeneouslydistributed over the entire layer.

For a definition of the terms partially cured and not cured, see belowin the section curing.

In principle, the layer wherein a fluid droplet is not cured before anadjacent droplet is subsequently applied may be located anywhere in thez dimension of the flexographic relief image, i.e. in the elastomericfloor, the optional mesa relief and the image relief. However, whenfluid droplets with a higher drop volume are used to make theelastomeric floor and/or the optional mesa relief as compared to thefluid droplets that are used to form the image relief, it is preferredthat the layer is the upper most layer of the elastomeric floor and/orthe optional mesa relief. The layer may also be the top layer of theimage relief, i.e. forming the print surface of the flexographic image.

More than one of the layers, for example two, three, four or more thanfive of the layers, wherein a fluid droplet is not cured before anadjacent droplet is subsequently applied may also be applied. Suchlayers may be applied on top of each other or they may be appliedindividually throughout the flexographic relief image, for example theupper most layer of the elastomeric floor and the upper most layer ofthe optional mesa relief. According to a particular preferred embodimentof the method according to the present invention, such layers arepreferably the upper most layer of the elastomeric floor and the uppermost layer of the mesa relief, or these layers are preferably the uppermost layer of the mesa relief and the upper most layer of the imagerelief, or these layers are the upper most layer of the elastomericfloor, the uppermost layer of the mesa relief and the upper most layerof the image relief.

As the flexographic printing master preferably comprises more than 20layers, it follows that for almost all layers each fluid droplet appliedis at least partially cured before an adjacent fluid droplet issubsequently applied, with the exception of preferably 1, 2 or 3 layerswherein a fluid droplet is not cured before an adjacent fluid droplet ofthe same layer is subsequently applied.

According to yet another preferred embodiment, the layer may be auniform layer applied on top of the flexographic image, i.e. coveringboth the printing and the non-printing areas.

The composition of all layers applied by inkjet may be the same ordifferent. For example, the composition of the curable fluid used toapply the floor and/or mesa relief may be the same or different to thecomposition of the curable fluid used to apply to image relief. Also,the composition of the curable fluid used to apply the layers wherein afluid droplet is not cured before an adjacent droplet is subsequentlyapplied may be the same or different to the composition of the fluiddroplets used to apply the other layers wherein a fluid droplet is atleast partially cured before an adjacent droplet is subsequentlyapplied. For example, the fluids used to apply the layers wherein afluid droplet is not cured before an adjacent droplet is subsequentlyapplied may be optimized to improve the coalescence between neighbouringdroplets or to improve the spreading of the applied droplets, in orderto further improve the flatness or evenness of the printing surface ofthe flexographic master.

It is believed that adjacent fluid droplets of the same layer which arenot cured may at least partially coalesce. When all fluid dropletsforming a layer coalesce, a homogeneous layer of fluid is formed. Whensuch a homogeneous layer of fluid is then cured, a smoother surface isobtained as compared with the situation where no substantial coalescenceof the drops of fluid occurs.

Flexographic Printing Support

Two forms of flexographic printing supports may be used: a sheet formand a cylindrical form, the latter commonly referred to as a sleeve. Ifthe print master is created as a sheet form on a flatbed inkjet device,the mounting of the sheet form on a print cylinder may introducemechanical distortions resulting in so-called anamorphic distortion inthe printed image. Such a distortion may be compensated by an anamorphicpre-compensation in an image processing step prior to halftoning.Creating the print master directly on a sheet form mounted on a printcylinder or directly on a sleeve avoids the problem of geometricdistortion altogether.

Using a sleeve as support provides improved registration accuracy andfaster change over time on press. Furthermore, sleeves may bewell-suited for mounting on an inkjet printer having a rotating drum, asshown in FIG. 3. Seamless sleeves have applications in flexographicprinting of continuous designs such as in wallpaper, decoration, giftwrapping paper and packaging.

The term “flexographic printing support”, often encompasses two types ofsupport:

-   -   a support without elastomeric layers on its surface; and    -   a support with one or more elastomeric layers on its surface.

These one or more elastomeric layers form the so-called elastomericfloor.

In a preferred embodiment of the method of the present invention, theflexographic printing support referred to is a support, preferably asleeve, without one or more elastomeric layers forming an elastomericfloor. Such a sleeve is also referred to as a basic sleeve or a sleevebase. Basic sleeves typically consist of composites, such as epoxy orpolyester resins reinforced with glass fibre or carbon fibre mesh.Metals, such as steel, aluminium, copper and nickel, and hardpolyurethane surfaces (e.g. durometer 75 Shore D) can also be used. Thebasic sleeve may be formed from a single layer or multiple layers offlexible material, as for example disclosed by US 2002466668. Flexiblebasic sleeves made of polymeric films can be transparent to ultravioletradiation and thereby accommodate backflash exposure for building afloor in the cylindrical printing element. Multiple layered basicsleeves may include an adhesive layer or tape between the layers offlexible material. Preferred is a multiple layered basic sleeve asdisclosed in U.S. Pat. No. 5,301,610. The basic sleeve may also be madeof non-transparent, actinic radiation blocking materials, such as nickelor glass epoxy. The basic sleeve typically has a thickness from 0.1 to1.5 mm for thin sleeves and from 2 mm to as high as 100 mm for othersleeves. For thick sleeves often combinations of a hard polyurethanesurface with a low-density polyurethane foam as an intermediate layercombined with a fiberglass reinforced composite core are used as well assleeves with a highly compressible surface present on a sleeve base.Depending upon the specific application, sleeve bases may be conical orcylindrical. Cylindrical sleeve bases are used primarily in flexographicprinting.

The basic sleeve or flexographic printing sleeve is stabilized byfitting it over a steel roll core known as an air mandrel or aircylinder. Air mandrels are hollow steel cores which can be pressurizedwith compressed air through a threaded inlet in the end plate wall.Small holes drilled in the cylindrical wall serve as air outlets. Theintroduction of air under high pressure permits to float the sleeve intoposition over an air cushion. Certain thin sleeves are also expandedslightly by the compressed air application, thereby facilitating thegliding movement of the sleeve over the roll core. Foamed adapter orbridge sleeves are used to “bridge” the difference in diameter betweenthe air-cylinder and a flexographic printing sleeve containing theprinting relief. The diameter of a sleeve depends upon the requiredrepeat length of the printing job.

Method of Applying an Elastomeric Floor on a Sleeve

According to another preferred embodiment of the present invention abasic sleeve provided with an elastomeric floor is prepared by applyingand curing fluid droplets thereby building up a plurality of layers offluid on top of each other so as to form the elastomeric floorcharacterized in that each fluid droplet applied is at least partiallycured before an adjacent fluid droplet is subsequently applied, with theexception that a fluid droplet applied during building up at least onelayer of fluid is not cured before an adjacent fluid droplet of the samelayer is subsequently applied. Preferably, the at least one layer is theupper most layer of the elastomeric floor. With this method, anelastomeric floor is obtained with a smooth surface, even when fluiddroplets having a large drop volume are used.

Such a sleeve provided with the elastomeric floor can then be furtherused to make a flexographic printing master by applying an optional mesarelief and an image relief by inkjet.

Apparatus for Creating the Flexographic Printing Master

Various preferred embodiments of an apparatus for creating theflexographic printing master by inkjet printing may be used. Inprinciple a flat bed printing device may be used, however, a drum basedprinting device is preferred. A particularly preferred drum basedprinting device using a sleeve body as flexographic support is shown inFIG. 1 and has been discussed in detail above.

Printhead

The inkjet printer includes any device capable of coating a surface bybreaking up a radiation curable fluid into small droplets which are thendirected onto the surface. In the most preferred embodiment theradiation curable fluids are jetted by one or more printing headsejecting small droplets in a controlled manner through nozzles onto aflexographic printing support, which is moving relative to the printinghead(s). A preferred printing head for the inkjet printing system is apiezoelectric head. Piezoelectric inkjet printing is based on themovement of a piezoelectric ceramic transducer when a voltage is appliedthereto. The application of a voltage changes the shape of thepiezoelectric ceramic transducer in the printing head creating a void,which is then filled with radiation curable fluid. When the voltage isagain removed, the ceramic returns to its original shape, ejecting adrop of fluid from the print head. However the inkjet printing method isnot restricted to piezoelectric inkjet printing. Other inkjet printingheads can be used and include various types, such as a continuous typeand thermal, electrostatic and acoustic drop on demand types. At highprinting speeds, the radiation curable fluids must be ejected readilyfrom the printing heads, which puts a number of constraints on thephysical properties of the fluid, e.g. a low viscosity at the jettingtemperature, which may vary from 25° C. to 110° C. and a surface energysuch that the printing head nozzle can form the necessary smalldroplets.

An example of a printhead according to a preferred embodiment of thecurrent invention is capable to eject droplets having a volume between0.1 and 100 picoliter (pl) and preferably between 1 and 30 pl. Even morepreferably the droplet volume is in a range between 1 pl and 8 pl. Evenmore preferably the droplet volume is only 2 or 3 pl.

Curing

For all layers of the relief image, except the at least one layer,immediately after the deposition of fluid droplet by the printhead thefluid droplet are exposed by a curing source. This providesimmobilization and prevents the droplets to run out, which woulddeteriorate the quality of the print master. Such curing of appliedfluid drops is often referred to as “pinning”.

Curing can be “partial” or “full”. The terms “partial curing” and “fullcuring” refer to the degree of curing, i.e. the percentage of convertedfunctional groups, and may be determined by, for example, RT-FTIR(Real-Time Fourier Transform Infra-Red Spectroscopy) which is a methodwell known to the one skilled in the art of curable formulations.Partial curing is defined as a degree of curing wherein at least 5%,preferably 10%, of the functional groups in the coated formulation orthe fluid droplet is converted. Full curing is defined as a degree ofcuring wherein the increase in the percentage of converted functionalgroups with increased exposure to radiation (time and/or dose) isnegligible. Full curing corresponds with a conversion percentage that iswithin 10%, preferably 5%, from the maximum conversion percentage. Themaximum conversion percentage is typically determined by the horizontalasymptote in a graph representing the percentage conversion versuscuring energy or curing time. When in the present application the term“no curing” is used, this means that less than 5%, preferably less than2.5%, most preferably less than 1%, of the functional groups in thecoated formulation or the fluid droplet is converted. In the methodaccording to a preferred embodiment of the present invention, appliedfluid droplets which are not cured are allowed to spread or coalescewith adjacent applied fluid droplets. Radiation curable fluids are curedby exposing them to actinic radiation, e.g. by UV curing, by thermalcuring and/or by electron beam curing. Preferably the curing process isperformed by UV radiation.

The curing source may be arranged in combination with the inkjetprinthead, travelling therewith so that the curable fluid is exposed tocuring radiation very shortly after having been jetted (see FIG. 1,curing source 150). It may be difficult to provide a small enoughradiation source connected to and travelling with the print head.Therefore, a static fixed radiation source may be employed, e.g. asource of UV-light, which is then connected to the printhead by aflexible radiation conductor such as a fibre optic bundle or aninternally reflective flexible tube.

Alternatively, a source of radiation arranged not to move with the printhead, may be an elongated radiation source extending transversely acrossthe flexographic printing support surface to be cured and parallel withthe slow scan direction of the print head (see FIG. 1, curing source170). With such an arrangement, each applied fluid droplet is cured whenit passes beneath the curing source 170. The time between jetting andcuring depends on the distance between the printhead and the curingsource 170 and the rotational speed of the rotating drum 140.

A combination of both curing sources 150 and 170 can also be used asdepicted in FIG. 1.

Any UV light source, as long as part of the emitted light can beabsorbed by the photo-initiator or photo-initiator system of the fluiddroplets, may be employed as a radiation source, such as, a high or lowpressure mercury lamp, a cold cathode tube, a black light, anultraviolet LED, an ultraviolet laser, and a flash light.

For curing the inkjet printed radiation curable fluid, the imagingapparatus preferably has a plurality of UV light emitting diodes. Theadvantage of using UV LEDs is that it allows a more compact design ofthe imaging apparatus.

UV radiation is generally classified as UV-A, UV-B, and UV-C as follows:

-   -   UV-A: 400 nm to 320 nm    -   UV-B: 320 nm to 290 nm    -   UV-C: 290 nm to 100 nm

The most important parameters when selecting a curing source are thespectrum and the intensity of the UV-light. Both parameters affect thespeed of the curing. Short wavelength UV radiation, such as UV-Cradiation, has poor penetration and enables to cure droplets primarilyon the outside. A typical UV-C light source is low pressure mercuryvapour electrical discharge bulb. Such a source has a wide spectraldistribution of energy, but with a strong peak in the short wavelengthregion of the UV spectrum. Long wavelength UV radiation, such as UV-Aradiation, has better penetration properties. A typical UV-A source is amedium or high pressure mercury vapour electrical discharge bulb.Recently UV-LEDS have become commercially available which also emit inthe UV-A spectrum and that have the potential to replace gas dischargebulb UV sources. By doping the mercury gas in discharge bulb with ironor gallium, an emission can be obtained that covers both the UV-A andUV-C spectrum. The intensity of a curing source has a direct effect oncuring speed. A high intensity results in higher curing speeds.

The curing speed should be sufficiently high to avoid oxygen inhibitionof free radicals that propagate during curing. Such inhibition not onlydecreases curing speed, but also negatively affects the conversion ratioof monomer into polymer. To minimize such oxygen inhibition, the imagingapparatus preferably includes one or more oxygen depletion units. Theoxygen depletion units place a blanket of nitrogen or other relativelyinert gas (e.g. CO₂), with adjustable position and adjustable inert gasconcentration, in order to reduce the oxygen concentration in the curingenvironment. Residual oxygen levels are usually maintained as low as 200ppm, but are generally in the range of 200 ppm to 1200 ppm.

Another way to prevent oxygen inhibition is the performance of a lowintensity pre-exposure before the actual curing.

A partially cured fluid droplet is solidified but still containsresidual monomer. This approach improves the adhesion properties betweenthe layers that are subsequently printed on top of each other. Partialintermediate curing is possible with UV-C radiation, UV-A radiation orwith broad spectrum UV radiation. As mentioned above, UV-C radiationcures the outer skin of a fluid droplet and therefore a UV-C partiallycured fluid droplet will have a reduced availability of monomer in theouter skin and this negatively affects the adhesion between neighbouringlayers of the relief image. It is therefore preferred to perform thepartial curing with UV-A radiation.

A final post curing however is often realized with UV-C light or withbroad spectrum UV light. Final curing with UV-C light has the propertythat the outside skin of the print master is fully hardened.

Thermal curing can be performed image-wise e.g. by use of a thermal heador a laser beam. If a laser beam is used, then preferably an infraredlaser is used in combination with an infrared dye in the curable fluid.When electron beams are employed, the exposure amount of the electronbeam is preferably controlled to be in the range of 0.1-20 Mrad.

It is important to avoid that light—even stray light—from a curingsource reaches the nozzles of a printhead, because this would cause thefluid to polymerize in the nozzles, resulting in “nozzle failure” or“clogging”. For this reason, a curing source and a printhead should besufficiently spaced apart, or a screen should be placed in between both.

Radiation Curable Fluids

The radiation curable fluid is preferably curable by actinic radiationwhich can be UV light, IR light or visible light. Preferably theradiation curable fluid is a UV curable fluid. The radiation curablefluid preferably contains at least a photo-initiator and a polymerizablecompound. The polymerizable compound can be a monofunctional orpolyfunctional monomer, oligomer or pre-polymer or a combinationthereof. The radiation curable fluid may be a cationically curable fluidbut is preferably a free radical curable fluid. The free radical curablefluid preferably contains substantially acrylates rather thanmethacrylates for obtaining a high flexibility of the applied layer.Also the functionality of the polymerizable compound plays an importantrole in the flexibility of the applied layer. Preferably a substantialamount of monofunctional monomers and oligomers are used.

In a preferred embodiment of the present invention, the radiationcurable fluid includes a photoinitiator and a polymerizable compoundselected from the group consisting of lauryl acrylate,polyethyleneglycol diacrylate, polyethylene glycol dimethacrylate,2-(2-ethoxyethoxy)ethyl acrylate, 2-phenoxyethyl acrylate,2-phenoxyethyl methacrylate, propoxylated neopentylglycol diacrylate,alkoxylated hexanediol diacrylate, isobornylacrylate, isodecyl acrylate,hexane diol diacrylate, caprolacton acrylate and urethane acrylates. Ina more preferred embodiment of the present invention, the radiationcurable fluid includes an aliphatic urethane acrylate. Aromatic typeurethane acrylates are less preferred. In an even more preferredembodiment, the urethane acrylate is a urethane monoacrylate. Commercialexamples include GENOMER™ 1122 and EBECRYL™ 1039. The flexibility of agiven urethane acrylate can be enhanced by increasing the linearmolecular weight between crosslinks. Polyether type urethane acrylatesare for flexibility also more preferred than polyester type urethaneacrylates. Preferably the radiation curable fluid does not include aminemodified polyether acrylates which reduce the flexibility of the curedlayer. An elastomer or a plasticizer is preferably present in theradiation curable fluid for improving desired flexographic propertiessuch as flexibility and elongation at break.

The radiation curable fluid may contain a polymerization inhibitor torestrain polymerization by heat or actinic radiation.

The radiation curable fluid may contain at least one surfactant forcontrolling the spreading of the fluid. The radiation curable fluid mayfurther contain at least one colorant for increasing contrast of theimage on the flexographic printing master.

The radiation curable fluid may further contain at least one acidfunctionalized monomer or oligomer. The radiation curable fluidpreferably has a viscosity at a shear rate of 100 s⁻¹and at atemperature between 15 and 70° C. of not more than 100 mPa·s, preferablyless than 50 mPa·s, and more preferably less than 15 mPa·s.

Monofunctional Monomers

Any polymerizable monofunctional monomer commonly known in the art maybe employed. Particular preferred polymerizable monofunctional monomersare disclosed in paragraphs [0054] to [0058] of EP-A 1637926 A. Two ormore monofunctional monomers can be used in combination. Themonofunctional monomer preferably has a viscosity smaller than 30 mPa·sat a shear rate of 100 s⁻¹ and at a temperature between 15 and 70° C.

Polyfunctional Monomers and Oligomers

Any polymerizable polyfunctional monomer and oligomer commonly known inthe art may be employed. Particular preferred polyfunctional monomersand oligomers are disclosed in paragraphs [0059] to [0063] of EP-A1637926. Two or more polyfunctional monomers and/or oligomers can beused in combination. The polyfunctional monomer or oligomer preferablyhas a viscosity larger than 50 mPa·s at a shear rate of 100 s⁻¹ and at atemperature between 15 and 70° C.

Acid Functionalized Monomers and Oligomers

Any polymerizable acid functionalized monomer and oligomer commonlyknown in the art may be employed. Particular preferred acidfunctionalized monomers and oligomers are disclosed in paragraphs [0066]to [0070] of EP-A 1637926.

Photo-Initiators

The photo-initiator, upon absorption of actinic radiation, preferablyUV-radiation, forms free radicals or cations, i.e. high-energy speciesinducing polymerization and crosslinking of the monomers and oligomersin the radiation curable fluid.

A preferred amount of photo-initiator is 1 to 10% by weight, morepreferably 1 to 7% by weight, of the total radiation curable fluidweight. A combination of two or more photo-initiators may be used. Aphoto-initiator system, comprising a photo-initiator and a co-initiator,may also be used. A suitable photo-initiator system comprises aphoto-initiator, which upon absorption of actinic radiation forms freeradicals by hydrogen abstraction or electron extraction from a secondcompound, the co-initiator. The co-initiator becomes the actualinitiating free radical.

Irradiation with actinic radiation may be realized in two steps, eachstep using actinic radiation having a different wavelength and/orintensity. In such cases it is preferred to use 2 types ofphoto-initiators, chosen in function of the different actinic radiationused. Suitable photo-initiators are disclosed in paragraphs [0077] to[0079] of EP-A 1637926.

Inhibitors

Suitable polymerization inhibitors include phenol type antioxidants,hindered amine light stabilizers, phosphor type antioxidants,hydroquinone monomethyl ether commonly used in (meth)acrylate monomers,and hydroquinone, methylhydroquinone, t-butylcatechol, pyrogallol mayalso be used. Of these, a phenol compound having a double bond inmolecules derived from acrylic acid is particularly preferred due to itshaving a polymerization-restraining effect even when heated in a closed,oxygen-free environment. Suitable inhibitors are, for example,SUMILIZER™ GA-80, SUMILIZER™ GM and SUMILIZER™ GS produced by SumitomoChemical Co., Ltd.

Since excessive addition of these polymerization inhibitors will lowerthe sensitivity to curing of the radiation curable fluid, it ispreferred that the amount capable of preventing polymerization bedetermined prior to blending. The amount of a polymerization inhibitoris generally between 200 and 20,000 ppm of the total radiation curablefluid weight.

Oxygen Inhibition

Suitable combinations of compounds which decrease oxygen polymerizationinhibition with radical polymerization inhibitors are:2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butane-1 and1-hydroxy-cyclohexyl-phenyl-ketone; 1-hydroxy-cyclohexyl-phenyl-ketoneand benzophenone;2-methyl-1[4-(methylthio)phenyl]-2-morpholino-propane-1-on anddiethylthioxanthone or isopropylthioxanthone; and benzophenone andacrylate derivatives having a tertiary amino group, and addition oftertiary amines. An amine compound is commonly employed to decrease anoxygen polymerization inhibition or to increase sensitivity. However,when an amine compound is used in combination with a high acid valuecompound, the storage stability at high temperature tends to bedecreased. Therefore, specifically, the use of an amine compound with ahigh acid value compound in ink-jet printing should be avoided.Synergist additives may be used to improve the curing quality and todiminish the influence of the oxygen inhibition. Such additives include,but are not limited to ACTILANE™ 800 and ACTILANE™ 725 available fromAKZO NOBEL, EBECRYL™ P115 and EBECRYL™ 350 available from UCB CHEMICALSand CD 1012, CRAYNOR™ CN 386 (amine modified acrylate) and CRAYNOR™ CN501 (amine modified ethoxylated trimethylolpropane triacrylate)available from CRAY VALLEY. The content of the synergist additive is inthe range of 0 to 50% by weight, preferably in the range of 5 to 35% byweight, based on the total weight of the radiation curable fluid.

Plasticizers

Plasticizers are usually used to improve the plasticity or to reduce thehardness of adhesives, sealing compounds and coating compositions.Plasticizers are fluid or solid, generally inert organic substances oflow vapour pressure. Suitable plasticizers are disclosed in paragraphs[0086] to [0089] of EP-A 1637926. The amount of plasticizer ispreferably at least 5% by weight, more preferably at least 10% byweight, each based on the total weight of the radiation curable fluid.The plasticizers may have molecular weights up to 30,000 but arepreferably fluids having molecular weights of less than 5,000.

Elastomers

The elastomer may be a single binder or a mixture of various binders.The elastomeric binder is an elastomeric copolymer of a conjugateddiene-type monomer and a polyene monomer having at least twonon-conjugated double bonds, or an elastomeric copolymer of a conjugateddiene-type monomer, a polyene monomer having at least two non-conjugateddouble bonds and a vinyl monomer copolymerizable with these monomers.Preferred elastomers are disclosed in paragraphs [0092] and [0093] ofEP-A 1637926.

Surfactants

The surfactant(s) may be anionic, cationic, non-ionic, or zwitter-ionicand are usually added in a total quantity below 20% by weight, morepreferably in a total quantity below 10% by weight, each based on thetotal radiation curable fluid weight.

A fluorinated or silicone compound may be used as a surfactant, however,a potential drawback is bleed-out after image formation because thesurfactant does not cross-link. It is therefore preferred to use acopolymerizable monomer having surface-active effects, for example,silicone-modified acrylates, silicone modified methacrylates,fluorinated acrylates, and fluorinated methacrylates.

Colorants

Colorants may be dyes or pigments or a combination thereof. Organicand/or inorganic pigments may be used.

Suitable dyes and pigments include those disclosed by ZOLLINGER,Heinrich, Color Chemistry: Syntheses, Properties, and Applications ofOrganic Dyes and Pigments,3rd edition, WILEY-VCH, 2001, ISBN 3906390233,page.550. Suitable pigments are disclosed in paragraphs [0098] to [0100]of EP-A 1637926. The pigment is present in the range of 0.01 to 10% byweight, preferably in the range of 0.1 to 5% by weight, each based onthe total weight of radiation curable fluid.

Solvents

The radiation curable fluid preferably does not contain an evaporablecomponent, but sometimes, it can be advantageous to incorporate anextremely small amount of a solvent to improve adhesion to theink-receiver surface after UV curing. In this case, the added solventmay be any amount in the range of 0.1 to 10.0% by weight, preferably inthe range of 0.1 to 5.0% by weight, each based on the total weight ofradiation curable fluid.

Humectants

When a solvent is used in the radiation curable fluid, a humectant maybe added to prevent the clogging of the nozzle, due to its ability toslow down the evaporation rate of radiation curable fluid. Suitablehumectants are disclosed in paragraph [0105] of EP-A 1637926. Ahumectant is preferably added to the radiation curable fluid formulationin an amount of 0.01 to 20% by weight of the formulation, morepreferably in an amount of 0.1 to 10% by weight of the formulation.

Biocides

Suitable biocides include sodium dehydroacetate, 2-phenoxyethanol,sodium benzoate, sodium pyridinethion-1-oxide, ethyl p-hydroxy-benzoateand 1,2-benzisothiazolin-3-one and salts thereof. A preferred biocidefor the radiation curable fluid suitable for the method formanufacturing a flexographic printing master according to a preferredembodiment of the present invention, is PROXEL™ GXL available fromZENECA COLOURS.

A biocide is preferably added in an amount of 0.001 to 3% by weight,more preferably in an amount of 0.01 to 1.00% by weight, each based onradiation curable fluid.

Preparation of Radiation Curable Fluids

The radiation curable fluid may be prepared as known in the art bymixing or dispersing the ingredients together, optionally followed bymilling, as described for example in paragraphs [0108] and [0109] ofEP-A 1637926.

EXAMPLES

Materials

All materials used in the examples were readily available from standardsources such as Aldrich Chemical Co. (Belgium) and Acros (Belgium)unless otherwise specified.

-   -   DPGDA is a dipropylene glycol diacrylate available from UCB    -   Agfarad is a mixture of 4 wt. % p-methoxyphenol, 10 wt. %        2,6-di-tert-butyl-4-methylfenol and 3.6 wt. % Aluminium        N-nitroso-phenylhydroxylamine (available from CUPFERRON AL) in        DPGDA    -   Ebecryl 1360 is a silicone hexa-acrylate available from Cytec    -   SR506D is an isobornylacryate available from Sartomer    -   Genomer 1122 is a low viscous monofunctional urethane        acrylate(2-acrylic acid        2-(((acryl-amino)carbonyl)oxy)ethylester) from RAHN    -   SR610 is a polyethylene glycol (600) diacrylate available from        Sartomer    -   Darocur TPO is a 2,4,6-Trimethylbenzoylphenyl-phosphineoxide        from CIBA    -   Santicizer 278 is a plasticizer available from MONSANTO    -   Genocure EPD is a Ethyl-4-dimethyl-aminobenzoate available from        RAHN    -   Darocur ITX is an isopropylthioxanthone available from CIBA

Example 1

As flexographic support a subbed PET film having a thickness of 100 μmwas used. The support was mounted on a drum. The rotation speed of thedrum during printing was 30 cm/s. As curing source, a UV LED ((365 nm)array encompassing the full print width was used. The distance betweenthe UV LED array and the drum was 2 cm, the distance between the UV LEDarray and the printhead approximately 8 cm. Taking into account therotational speed of the drum this means that the time between applying adrop of fluid on the support or on previous applied layers of fluid andcuring that drop of fluid was approximately 0.25 s. The energy output ofthe UV LEDs was 0.65 W/cm² (corresponding with a UV-LED power controllersetting of 1.5 A). As printhead, an Agfa UPH printhead was used having700 nozzles with a nozzle diameter of 25 μm resulting in a drop volumeof 8 pl.

56 layers of ink were printed in one pass of the printhead during whichthe UV-LED array was active, resulting in a thickness of approximately270 μm. The layers were printed in such a way that each applied drop ofink was at least partially cured before an adjacent drop of ink wassubsequently applied (using the method as described on page 7 and 8 ofthe description and in EP-A 2199066).

Example 2

In example 2, on top of the 56 layers printed as in example 1, twoadditional layers, having the same composition as those of the 56previous applied layers, were printed in one additional second pass ofthe printhead during which the UV-LED array was not active. Therefore,each drop of ink applied in these two layers was not cured before anadjacent drop of ink of the same layer was subsequently applied. As aresult, adjacent applied drops of ink at least partially coalesced. 1minute after completion of the second pass of the printhead, the UV-LEDarray was activated to cure the last two layers applied. The energyoutput of the UV-LEDs was 6 W/cm² (corresponding with a UV-LED powercontroller setting of 15 A).

Roughness measurements were carried out on both examples 1 and 2. Themeasurements were carried out in accordance with ISO4288 with a Dektak-8stylus profiler, available from VEECO using a needle having a tip radiusof 2.5 μm, a cone angle of 45° and a static measuring force of 5 mg. InTable 1, each value is an average of 10 measurements.

TABLE 1 Ra (um) Rv (um) Rp (um) Rt (um) Rz (um) Ex. 1 (COMP) 0.224−0.929 2.102 3.031 1.536 Ex. 2 (INV) 0.055 −0.215 0.184 0.399 0.283

The viscosity of the ink (measured with a Brookfield DV-II viscosimeterat 45° C.) amounted to 10.80 mPa·s. The static surface tension, measuredwith a “Tensiometer K9” from Krüss was 28.90 mN/m.

The ink used had the composition as shown in Table 2.

TABLE 2 Ingredient Amount wt. % SR506D 42.2 Genomer 1122 13.33 SR61017.76 Santicizer 278 11.10 Ebecryl 1360 0.04 Agfarad 0.70 Genocure EPD5.00 Darocur ITX 5.00 Darocur TPO 4.90

It is clear from the roughness parameters shown in Table 1 that theprinting surface of the inventive example 2 is significantly smootherthan the printing surface of the comparative example 1.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

The invention claimed is :
 1. A method of preparing a flexographicprinting master including a relief on a flexographic printing support,the method comprising: building up a plurality of layers by applying andcuring fluid droplets on a flexographic printing support, each of thefluid droplets being cured or partially cured before an adjacent one ofthe fluid droplets within a same layer of the plurality of layers issubsequently applied; and building up at least one layer of theplurality of layers by the steps of: a) applying a fluid droplet; b)applying a fluid droplet in the at least one layer adjacent to the fluiddroplet applied in step a) without first curing or partially curing thefluid droplet applied in step a); c) allowing the fluid droplet appliedin step a) to at least partially coalesce with the fluid droplet appliedin step b); and d) curing the fluid droplets at least partiallycoalesced in step c).
 2. The method for preparing a flexographicprinting master according to claim 1, wherein at least 75% of the fluiddroplets applied during the build up of the at least one layer are notcured before the adjacent fluid droplet of the same layer issubsequently applied.
 3. The method for preparing a flexographicprinting master according to claim 1, wherein the relief includes a mesarelief and an image relief.
 4. The method for preparing a flexographicprinting master according to claim 3, wherein the at least one layer isan upper most layer of the mesa relief.
 5. The method for preparing aflexographic printing master according to claim 3, wherein the at leastone layer is an upper most layer of the image relief.
 6. The method forpreparing a flexographic printing master according to claim 1, whereinthe fluid droplets are UV curable.
 7. The method for preparing aflexographic printing master according to claim 1, wherein theflexographic printing support is a sleeve.
 8. The method for preparing aflexographic printing master according to claim 1, wherein theflexographic printing support includes an elastomeric layer.
 9. Themethod for preparing a flexographic printing master according to claim8, wherein the flexographic printing support is a sleeve.
 10. The methodfor preparing a flexographic printing master according to claim 1,wherein the relief includes a top hat segment.
 11. The method forpreparing a flexographic printing master according to claim 1, whereinthe steps a), b), and c) are repeated at least once before performingstep d).
 12. The method for preparing a flexographic printing masteraccording to claim 3, wherein the at least one layer built up by thesteps a) to d) is an upper most layer of the mesa relief.
 13. The methodfor preparing a flexographic printing master according to claim 1,wherein the at least one layer built up by the steps a) to d) is anupper most layer of the relief.
 14. The method for preparing aflexographic printing master according to claim 1, wherein all of thefluid droplets applied during the build up of the at least one layer arenot cured before the adjacent fluid droplet of the same layer issubsequently applied.